Fork lift trucks commonly have a vertically extendable mast assembly wherein a fork carriage is movable relative to an inner mast section which is in turn reciprocably mounted within an outer mast assembly. The fork carriage is raised by a hydraulic jack through a chain and pulley arrangement while the inner mast section is raised directly by the hydraulic jack. To reduce the length of the chain, some of the mast assemblies include an elongated chain rod having one end pivotally attached to the chain and its other end anchored to the stationary mast section. The hydraulic jack and chain rod are normally loosely mounted within the mast assembly to permit a slight degree of freedom to accommodate for the looseness in the mast assembly as well as any deformation occurring in the mast assembly when supporting a load. One of the problems frequently encountered with such an arrangement occurs when the fork carriage is being lowered to place a load on an elevated loading dock, tote tub, and the like. If the hydraulic jack is allowed to retract after movement of the fork carriage is stopped in a elevated position, the inner mast section will continue to move downwardly and the lift chain becomes slack. Occasionally, the chain and chain rod will buckle toward the truck and become entrapped beneath a cross brace of the downwardly moving inner mast section. This causes binding and malfunctioning of the mast assembly and leads to failure in the chain mechanism.